What Went Wrong in the 777 Crash in San Francisco?

In a press conference Sunday, officials announced that the Instrument Landing System that should have helped the Asiana Airlines 777 pilots land at San Francisco International Airport was down. But that doesn't explain why the plane crashed on a clear day.

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In good weather, pilots typically have the option of guiding an aircraft to the ground by locking onto ultra-high-frequency radio signals broadcast from the target runway. Up and down corrections can be made either by hand or by an autopilot computer, keeping the plane descending on the proper glide slope. GPS systems are also aboard to help the pilots stay on course.

But in the case of Saturday's Asiana Airlines crash of a Boeing 777 at San Francisco International Airport, the pilots didn't have the option of following a glide slope set by such an Instrument Landing System (ILS). Deborah Hersman, chairman of the National Transportation Safety Board, told reporters at a Sunday afternoon briefing that the ILS for runway 28L at SFO where the 777 was supposed to land has been down since June 1 due to an airport construction project scheduled to run through Aug. 2.

Still, the ILS system is supposed to be purely optional in good weather. The plane had a GPS system aboard. And the skies were so clear that investigators seem mystified as to how the pilots could not have known until the last moments that they were coming down short of the runway. Hersman said whether pilots were "using automation or whether they were hand flying is information that we have yet to determine."

The Asiana Airlines flight from Seoul ended with a furious attempt to save the plane. The pilots gave no indication of trouble until seven seconds before impact, Hersman said, when the cockpit voice recorder shows a crewmember calling for more speed—the data recorder shows that the plane was flying "significantly below" the intended 137-knot approach speed. A stick shaker vibration was sent into the control yokes to warn the pilots that they were approaching aerodynamic stall. At 1.5 seconds, a call for a go-around was heard, but it was too late. Passengers reportedly heard the plane's engines roaring just as the jet hit the seawall short of the runway.

It will be up to investigators to figure out why, on a clear day, the Asiana flight ended as it did: with two dead and hundreds injured on a plane that skidded and spun to a stop before catching fire.

Hersman stopped short of blaming the crew for steering the 777 into the seawall at the end of runway 28L, but in the press conference she sounded mystified. She noted that visual flight rules were in effect during the approach, meaning the conditions were so clear that pilots should have been able to land solely by sight. In addition, Hersman said an optical Precision Approach Path Indicator was functioning. The NTSB has yet to hear the pilots' side of the story, however. Hermsan said the board hopes to interview them in the next few days.

It's early in the investigation, but aviation lawyer Mary Schiavo tells PopMech that she wonders if this accident could turn out to be a cautionary tale about the need for crews to stay sharp with stick and rudder operations even in a high-tech age.

"Perhaps old-fashioned flying was the only thing they could rely on, but nobody's done it for a very long time," she says.

As odd as it sounds, the clear, sunny skies might have deceived the pilots, Schiavo says. Planes tend to sink faster in warm, moist air over the water, and pilots flying by hand must take that into account.

The two deaths from the crash landing appear to be the first for a 777, which made its commercial debut in June 1995. The website Airsafe.com, which tracks air safety for flyers, reported no passenger fatalities for the 777s before yesterday.

"Pilots use ILS virtually all the time—good weather, bad weather," said Lyle Schaefer, a former Navy pilot and independent accident investigator who sometimes works with Schiavo. And though the pilots shouldn't have needed it to land, he cautioned against jumping to conclusions at this time.

"In this case, the airplane hit the seawall with the landing gear short of the runway, and knocked engines off, knocked the tail off the airplane, which is pretty amazing that anybody survived that. But we don't know why. There's a myriad of things that could enter into that equation."